1. Field of the Invention
The present invention relates to a zoom lens system, and more particularly to a zoom lens system suitably used in an image taking optical system in a video camera, a digital still camera, or the like.
2. Related Background Art
In recent years, with improvement in a function of an image pickup apparatus using a solid-state image pickup element, such as a video camera or a digital still camera, a compact zoom lens system having a high resolution has been desired as an image taking optical system used for the image pickup apparatus.
In general, a lens type of three to five units has been known as a zoom lens used for the image pickup apparatus (optical apparatus) using the solid-state image pickup element such as a CCD sensor of a video camera, a digital still camera, or the like (for example, Japanese Patent Application Laid-Open No. S63-081313 (corresponding to U.S. Pat. No. 4,802,747) and Japanese Patent Application Laid-Open No. H03-296706). The lens type includes at least a lens unit with positive refracting power, a lens unit with negative refracting power, and a lens unit with positive refracting power. Of these lens units, a lens unit nearest an object is fixed during zooming.
On the other hand, an optical system in which the entire lens length is extremely short, a view angle is wide in view of a property of a still image, and an optical performance is higher than that in a zoom lens used for a moving image video camera has been desired as a zoom lens for a digital still camera for taking the still image using a solid-state image pickup element.
A zoom lens which includes a lens unit with negative refracting power and a lens unit with positive refracting power has been known as a lens system which has a wide angle range, is bright, and obtains a high performance even when a zoom ratio is a relatively low zoom ratio of 2.5 to 3 (for example, Japanese Patent Publication No. H06-066008 (corresponding to U.S. Pat. No. 4,662,723)). With this zoom lens, zooming is performed by changing an air interval between the respective lens units.
There has been known a zoom lens which includes a first lens unit with negative refracting power, a second lens unit with positive refracting power, and a third lens unit with positive refracting power and in which an interval between the second lens unit and the third lens unit increases during zooming from a wide angle end to a telephoto end (for example, Japanese Patent Publication No. H07-052256 (corresponding to U.S. Pat. No. 4,733,952)).
Also, there has been known a zoom lens which includes a first lens unit with negative refracting power, a second lens unit with positive refracting power, and a third lens unit with positive refracting power and in which an interval between the second lens unit and the third lens unit reduces during zooming from a wide angle end to a telephoto end (for example, U.S. Pat. No. 5,434,710).
Also, there has been known a zoom lens which includes a first lens unit with negative refracting power, a second lens unit with positive refracting power, a third lens unit with positive refracting power, and a fourth lens unit with positive refracting power and in which an interval between the second lens unit and the third lens unit reduces during zooming from a wide angle end to a telephoto end, and the fourth unit is fixed during zooming (for example, Japanese Patent Application Laid-Open No. S60-031110).
Also, there has been known a zoom lens which includes a first lens unit with negative refracting power, a second lens unit with positive refracting power, a third lens unit with positive refracting power, which are disposed in order from an object side (for example, Japanese Patent Application Laid-Open Nos. H10-213745 and 2001-100098). The first lens unit includes three or more lenses and has at least one aspherical surface.
In recent years, with reduction in size of the image pickup apparatus and increase in the number of pixels of the image pickup element, a zoom lens which has a high optical performance and which is small in the entire lens system has been desired as a zoom lens used for a digital still camera, a video camera, and the like. In addition, it has been desired to record a high quality still image by the video camera. Therefore, a lens system which has a high optical performance but is small is required.
In general, with respect to a negative lead type zoom lens, in order to reduce the number of lenses in the entire lens system to simplify a lens structure and in order to obtain a preferable optical performance over the entire zoom range while attempting to widen a view angle, it is necessary to suitably set refracting power to each of the lens units, a lens structure of each of the lens units, a surface provided as an aspherical surface if the aspherical surface is used, and the like.
When the selections of the refracting power to each of the lens units, the lens structure, and the surface provided as the aspherical surface are unsuitable, an effect obtained by providing the aspherical surface is small. Therefore, a variation in aberration accompanying zooming becomes larger, so that it is hard to obtain a high optical performance over the entire zoom range.